Accurate assessment of a dialysis patient's hydration status and prediction of dry body weight (DW or dry weight) is a major problem in the clinical management of the dialysis patient. In both hemodialysis and peritoneal dialysis patients, dry weight is the target weight at the end of dialysis treatment which best reflects removal of excess water from the body. In clinical practice, estimation of DW is an imprecise undertaking, and depends to a large extent on the treating physician's interpretation, based on his or her medical experience and familiarity with the particular patient's condition, of clinical symptoms and signs such as changes in the blood pressure, pulse, and weight of the patient. The correct interpretation of such signs and symptoms is complicated by the fact that the pre-treatment body weight varies for each treatment, the amount of excess fluid is not constant and the amount of fluid that can or should be removed from any particular patient during any particular dialysis treatment may be limited by an individual's cardiovascular tolerance, often manifested by clinical signs and symptoms, such as pretibial edema, dyspnea, cramps and/or a decline in blood pressure. Alternatively, an overestimation of the amount of fluid to be removed may result in potentially avoidable symptoms, unnecessarily lengthy dialysis treatments and often prolonged stays at the dialysis facility. Therefore, over- or underestimation of DW will significantly affect both the efficiency of dialysis treatment and patients' quality of life.
Bioelectrical impedance analysis (BIA) has been recognized as a noninvasive and simple technique to measure body hydration and hydration status (i.e. over-, under- or normal hydration) of subjects for more than twenty years. There is substantial literature on using BIA for the study of dry weight. Kouw et al proposed a method to measure changes in regional conductivity, and then to measure regional extracellular volume (ECV) and intracellular volume (ICV) by BIA. See, P. M. Kouw, et al., Assessment of post-dialysis dry weight: an application of the conductivity measurement method. Kidney Int. 41:440-444,1992. However, Kouw's method cannot be used to measure interstitial fluid alone as it does not distinguish between interstitial fluid and plasma, both of which make up the ECV compartment. Piccoli published a method of BIA vector analysis which uses the ratio of resistance to reactance to identify dry weight. While this technique could be used to compare the subjects' body hydration, it is unable to predict individual patient's dry weight because of the significant variation in measured values. See, Piccoli A: Identification of operational clues to dry weight prescription in hemodialysis using bioimpedance vector analysis. Kidney Int. 5 3:1036-1043,1998.
Recently, there have been increased numbers of dry weight studies using blood volume (BV) measurements. See, for example, J. P. de Vries et al, Non-invasive monitoring of blood volume during hemodialysis: Its relation with post-dialytic dry weight. Kidney Int 44:851-854,1993, and J. K. Leypold, et al, Determination of circulating blood volume by continuously monitoring hematocrit during hemodialysis. J. Am. Soc. Nephrol. 6:214-219,1995. Blood volume measurement is a noninvasive technique that can be used to indicate water concentration in blood, i.e. hematocrit, during hemodialysis, but it cannot be used to directly determine dry weight because changes in blood volume are mainly dependent on the rate of vascular refilling which, in part, is independent of body hydration. See, e.g., J. K. Leypoldt, et al, Evaluating volume status in hemodialysis patients. Adv. Ren. Replace. Ther. 5:64-74, 1998. On the other hand, since a change in the hematocrit level may alter conductivity in the blood during dialysis, it is difficult to obtain information about tissue hydration by either traditional bioelectrical impedance analysis or blood volume analysis. To date, a major problem has been how to measure resistivity of blood and tissue separately, in order to estimate the fluid volume in the intravascular compartment and the interstitial compartment, respectively.
Thus, there is a need for a precise, easily used and operator independent method for determining the hydration status of a dialysis patient, identifying or predicting the dry weight of such a patient and calculating the amount of fluid that should be removed during a dialysis session.